Diagnostic audiometer



Dec. 7, 1965 A. HYMAN DIAGNOSTIC AUDIOMETER Filed July 12. 1962 INVEN TOR. ABQAHAM HyMA BY M... ATTOQM y mm mm Mi wt United States Patent DIAGNOSTIC AUDIOMETER Abraham Hyman, Nassau, N.Y., assignor to the United States of America as represented by the Secretary of the Army Filed July 12, 1962, Ser. No. 209,476 7 Claims. (Cl. 179--1) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the government for governmental purposes without the payment to me of any royalty thereon.

This application relates generally to improvements in the art of audiometers and relates more specifically to diagnostic audiometers of a type employing a new combination of circuit elements resulting in a type of audiometer circuit having a highly selective masking means accompanied by greater stability and reliability over a broad spectrum of desired audio frequencies.

A satisfactory audiometer must enable the physician to accurately and satisfactorily measure the hearing sensitivity and acuity of an individual patient over the principal frequency range of audible sound. The frequency range which is important for hearing, and in which the range of hearing sensitivity is greatest lies substantially between 100 and 8000 cycles per second, and an audiometer for determining hearing impairments of deafened persons should be able to measure eighty percent loss of hearing for this range of frequency. In addition, a satisfactory diagnostic audiometer should be able to selectively mask, diagnose and measure steps of audible sound frequencies within the testing range above a pure tone and the intensity of the tone should be accurately measurable. A change of sound intensity by one decibel is substantially the minimum change detectible by the human ear. However, because of the uncertainty of judgment of an average individual, one decibel (unit of hearing loss) change in sound intensity is of no great significance. However, for most practical purposes in testing the hearing of a deafened individual, it is sufiicient to test the individuals hearing with an audiometer capable of producing good tones at frequency steps of 250, 500, 1000, 2000, 3000, 4000, and 7000 cycles per second. However, pure tones at frequency steps which are an octave apart and extend over a range from 128 to 8192 cycles per second and which are capable of determining ones hearing ability in steps of five decibels may be used if desired.

The improvement of the circuit of this application over the art resides more specifically in the conversion of a screening type of audiometer circuit to a diagnostic audiometer circuit by the incorporating of a white noise generator having a random noise signal to be used as a selective and highly reliable masking means for a broad spectrum of audio frequencies for diagnostic purposes, and the circuit by way of improvements. further provides for incorporation of a Zener diode in the power circuit to be used for both a voltage control and as a means for presenting a relatively low impedance to the audiometer branch circuits permitting a variation or removal of a branch circuit without destroying circuit stability, together with a unique counterpoise ground system for the audiometer circuitry which substantially eliminates the possibility of shock hazard between the instrument panel and other ground sources. The circuit provides means to block out (or mask) the signal transferred via bone condition from the ear under test to the opposite ear of a patient.

The primary object of this invention is to provide a dual purpose audiometer circuit which is adapted to be used both as a diagnostic and screening type of audiometer.

Another object of this invention is to provide an audiometer circuit with a selective masking means which contributes to uniformity of spectrum amplitude for diagnostic purpose While maintaining greater stability and reliability of the circuit over a broad spectrum of desired audio frequencies for both an air conduction transducer and a bone conduction transducer.

Another object of this invention is to provide an improved electrical circuit for a diagnostic audiometer.

Another object of the invention is to provide an improved diagnostic audiometer circuit which employs a counterpoise ground means, which substantially solves the problem of complex ground loops when the measurement of low levels must be made (a few microvolts) in the proximity of strong electric and electromagnet fields.

The foregoing and other objects of the invention will be est understood from the following description of an exemplification thereof, reference being had to the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of the audiometer; and

FIG. 2 illustrates the filament circuit including a patient signal light and actuating switch arrangement for FIG. 1 in which the filament circuit is a series string arrangement with shunt-ing resistors for providing proper energizing filament current for the diagnostic audiometer.

The audiometer circuit of FIG. 1 is composed of a power supply including a Zener diode 12 to which is connected a phase-shift audio oscillator means 46 and accompanying frequency network including a calibrating means, the output of which is connected by an audio transformer and impedance matching network which is selectively connected to an attenuating type of step resistance ladder network, the output of which is further connected selectively to either an air induct-ion transducer or bone conduction transducer as desired. The masking means of the circuit is a white (or random) noise generator which is energized by the power supply and has an output connected by a selective switching means to the transducers to mask a desired frequency to be received by the transducers to prevent transferring the audio signal via bone conduction from the ear under test to the opposite ear of a patient.

FIG. 2 is a series string arrangement of circuit actuating switch 112, patient signal light 110, tube filaments 116 and 118 and indicating light 120. Resistances 113, 114 and 122 are shunting resistors for proper current flow as indicated. Filaments 116 and 118 are employed in the circuit to energize the cathode electrodes of tubes 46 and 39, respectively.

A voltage doubler power supply is used for energizing the audiometer circuit which consists, in general, of receptacle plug 3, switch 23, fuse 22 with heater element 21, rectifiers 6 and 5, capacitors 7, 8, 11 and 14, Zener diode 12, resistors 9, 10 and 13 and capacitor 4 as illustrated in the circuit of FIG. 1. During operation rectifier 6 conducts during one part of the AC. cycle and charges capacitor 4 to line voltage. During the second part of the AC. cycle, rectifier 5 conducts and charges capacitor 7 to line voltage. The potentials of capacitors 4 and 15 are effectively in series to give twice the line voltage. This voltage is further boosted to peak voltage by means of capacitor 8 which also serves as a filter with capacitor 11 and resistance 9. Resistor 13, and capacitor 14 serve for screen-grid filtering for DC. and signal voltages. Zener diode 12 is employed in the circuit as a voltage regulator and is operated to give a relatively low impedance. This characteristic is important and critical to the feasibility of this design. Tubes 39 and 40 may have impedances on the order of 5,000 ohms and each drain approximately 30 milliamperes of current. Switching plate voltage off or on to tube 46 by means of interrupter switches 32 and 33 causes a relatively large variation in impedance and current looking back to the power supply. Since Zener diode 12 presents a relatively low impedance to its branch circuits, a variation or removal of one branch circuit does not affect the other, thereby, maintaining circuit stability.

The phase shift oscillator circuit consists, generally of pentode tube 46, cathode bias network having resistance 47 and capacitor 48 coupled to counterpoise ground at point 49. The multiple frequency shift net-works for the oscillator are symbolically indicated by dotted outline closures a and b to i in connection with switches 25a and 25b. Each of the frequency networks have component resistors R1, R3, R4, component capacitors C1, C2, C3, calibrating resistance means R1 and each network includes a corresponding ground point 53 of the counterpoise ground system. The frequency network of the oscillator is selectively connected by switch 25:: and lead to grid 1a of pentode tube 46. The frequency control network is likewise selectively connected by switch 25b and lead 7c to plate 7a of tube 46 and primary winding of audio transformer 35 of FIG. 1. In the practice of the invention, tube 46 is a beam power tube which has a phase shift network associated with it as capacitors C1, C2, C3, and resistors R1, R2, R3, R4, RL and output transformer 35, and load stabilizing resistor 37, as illustrated in FIG. 1. Frequency levels are selectively calibrated by means of resistances R1, R2 and RL1 to RL9, as shown. In the calibration procedure for the oscillator the value of R1 and R2 are preset into each frequency level by manual adjustment and the impedance adjusting means for each frequency level is likewise preset into the audiometer circuit by resistive calibrating means RL1 and RL9 as shown in FIG. 1. Frequency and level switching for the earphones 82a and 82b, and bone conductor transducer 83 are accomplished selec-/ tively by means of multi-deck switch means 30 comprising switches 30a, 30b and 300.

The frequency of audio signal output of oscillator 46 may be varied in steps of frequency by condensers C1, C2, C3, and resistors R1, R2, R3, R4 and RL having selected values given in the following table:

It is to be understood, that the invention is not limited to the frequencies shown in the above table, as it is well within the ability of one skilled in the art to practice this invention by using other desirable frequencies.

The white noise generator of FIG. 1 consists, generally, of triode tube 39 and accompanying circuitry. In operation grid 1b of triode tube 39 is biased with respect to cathode 5b to cause current flow from cathode 5b to plate 712. This current flow causes ionization of the gas within the envelope of tube 39. The plate to cathode potential is on the order of 13 volts, below the ionization potential of the inert gases. However, the collision of electrons with the gas molecules results in an exchange of electrons causing ionization of the gas. The random motion of the gas modulates the cathode to plate electron stream to result in a random noise (or white noise) signal to appear between cathode and plate. The low plate voltage provides a saturated plate for stability. The low voltage and moderate current operation provides a moderately low circuit impedance on the order to several hundred ohms which establishes a low figure of merit or Q of an energy-storing system as defined at page 721 of The International Dictionary of Physics and Electronics, published by D. Van Nostrand Co., Inc., New York, New York (1956). This random noise generator has a broad banding effect which contributes to the uniformity of signal for a spectrum. Capacitor 38 is designed into the circuit to deliver maximum power to the primary. Resistor 3'7 and capacitor 34 form a decoupling network. Resistor 36 is a plate voltage dropping resistor. Resistor 40 is a biasing resistor to control cathode to plate current flow.

In. operation noise generator tube 39 is biased by means of resistor 40 for conduction with a low plate voltage. Capacitor 38 is for- DC. blocking and also forms part of the non-periodic noise oscillator with the primary winding of transformer 41. This oscillator contains a broad spectrum of frequencies. The low D.C. plate voltage contributes to the uniformity of the spectrum amplitude which is desirable and to its regulation since the tube is operating under saturated conditions. Transformer 41 is designed into the circuit for the non-periodic condition as well as for its ability to deliver maximum power to the secondary. The attenuation means in the secondary circuitry of transformer 41 is non-conventional due to the requirement that the resistive attenuation means be capable of attenuating a fairly linear wide level variation of 70 decibels. A special tapered potentiometer, L-pad, or T-pad were considered in connection with the above required characteristic and it was found that none of the conventional potentiometers or pads would give more than 3 decibels of the required variation of 70 decibels. The combination of shunting resistor 43 and 42 in series with linear potentiometer 44, gives the desirable characteristic.

The attenuator of the circuit comprises a resistive ladder network B having a certain desired decibel attenuation range in which the resistors may be arranged in series and series-parallel arrangement as shown for example by re sistances 68, 69 and 70 in FIG. 1.

The output of the phase shift oscillator tube 46 is inductively coupled by transformer 35 through selective switches 25c, 25d, 25e, 30 and S6 to the bridge attenuation network B as indicated in FIG. 1. Decks 25a and 25b switch in and out the RC frequency determining networks illustrated in the broken line boX a. The pure-tone audio output of transformer 35 is fed via wire connection 35b simultaneously to decks 25c, 25d, and 252 via series resistors RL through RL Deck 25c transfers the pure tone signal at the level predetermined by the value of the RL resistor in series with the deck to earphone 82b via contacts A on switch decks 30a and 30b and through the attenuator switch S6. The signal is transferred from the wiper arm of 250 to the A contact on 30a; the wiper arm of 30a transfers the signal to the db point on S6; the wiper arm of S6 transfers the signal to the wiper arm of 30b; in position A the Wiper arm of 30!) transfers the signal to earphone 82b.

Likewise, signal is transferred selectively to earphone 82a via switch deck 25d, contacts B on 30a and 30b, and S6. And likewise, signal is transferred to bone conductor 83 via switch deck 25e, contacts C and D on 30a and 30b, and S6. Deck 30c feeds white noise generator signal via wire connection 44b to one earphone while tone signal may be received on the second earphone. For example, when the wiper of 30c is on contact A white noise masking signal may be received on earphone 82a and tone is received on 82b. When 300 is on contact C, masking signal is received on 82b and tone is received on bone conductor 83; when 30c is on contact D masking is received on 82a and tone is received on bone conductor 83.

The output of the white noise generator is directly con nected by selective switches 30b and 30c to air conduction headphone 82a and 82b as shown in FIG. 1.

Member 82a indicates one air conductor transducer for one ear and member 82b indicates another air conducting transducer for another ear of a patient. Member 83 indicates a bone conducting receiver. One side of each of these transducers is selectively connected through switch 300 in series with switch S6 to a selected point of the bridge attenuator B as indicated for a desired decibel attenuation in FIG. 1.

Test selection for earphones 82a and 82b and bone conduetor 83, tone and masking is accomplished by multideck switch 30 comprising switches 30a, 30b and 300 as shown in FIG. 1.

Frequency and level switching for individual earphones 82a and 82b and bone conductor receiver 83 are accomplished by means of rnulti-deck switch 25 comprising individual switches 25a, 25b, 25c, 25d and 25e as shown in FIG. 1.

The counterpoise ground of this diagnostic audiometer circuit is used for the power supply, the tone generator and the masking generator circuitry. This eliminates the possibility of shock hazard between the front panel and other ground sources. A chassis ground is also provided on the secondary sides of the tone and masking generator transformer windings. The attenuator circuitry also uti- 'lizes this chassis grounding. This dual grounding system is a technique for minimizing effects due to complex ground loops. The problem of complex ground loops is important when the measurement of low levels must be made (a few microvolts) in proximity to strong electric and electromagnet fields.

The counterpoise ground system of this audiometer circuit is primarily intended as a safe system for providing an electrical return for a voltage doubler circuit. If ground points 35, 40b, 49 and 53 were returned to chassis ground, a shock hazard would exist when the power plug 3 was connected so that the hot side of the power line was connected to the chassis. The counterpoise gnound also serves to provide an isolated ground with respect to the output circuitry. A chassis ground 67a is also provided for the attenuator and for tone transformer 35 and white noise transformer 41. The output circuitry must be capable of providing low level signals accurately attenuated. The vacuum tube circuits and phase shift circuitry have signals at high amplitude and power levels which would be a source for high ground currents. However, these currents are isolated by virtue of the counterpoise ground.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

I claim:

1. An audiometer circuit for screening and diagnostic use comprising phase-shift oscillator means including a plurality of frequency determining networks, frequency masking generator means, audio transformer means connected to said oscillator means, step attenuating resistive network means selectively connected to said audio transformer means, a plurality of transducers selectively connected to said attenuation means, selective and interrupting switch means connecting said masking means to said selected transducer means to nullify the oscillator signal to a selected transducer during diagnosis and means for energizing said oscillator and masking means.

2. An audiometer circuit for diagnosing hearing impairments of personnel comprising an energizing circuit of substantially constant voltage, masking generator means, phase-shift oscillator means including separate fre quency determining circuit means for each frequency and having an audio signal output, resistive bridge attenuation means, plural transducer earphone means selectively connected to said bridge means, selective inductive coupling means interconnecting said bridge means and said oscillator means for screening selective audio fre quencies, and multiple switch means for selectively con necting said masking generator to said bridge and transducer means to selectively nullify the oscillator signal to a selected transducer during hearing diagnosis of a patient.

3. An audiometer circuit for diagnosing hearing impairments of personnel comprising energizing circuit means including a zener diode as a voltage regulation and impedance matching means, phase-shift oscillator means connected to said energizing circuit and including a separate frequency determining circuit means for each frequency and having an audio signal output, signal masking generator means connected to said energizing circuit, resistive attenuation bridge means for a selected decibel range, a plurality of conductive earphone transducers selectively connected to said attenuation means, inductive means operably connecting said oscillator means to said transducer means, counterpoise ground means connecting selected components of said audiometer circuit to maintain said selected components electrically above a reference ground to prevent electrical shock to a patient, multiple selective switch means interconnecting said transducers to said masking generator to block out the signal transferred by bone conduction from the ear under test to the opposite ear of a patient during diagnosis.

4. In a diagnostic audiometer circuit, a frequency generator adapted to produce alternating currents of predetermined frequencies including a number of frequency determining circuits energized by said generator, there being one circuit for each frequency, an attenuator including a resistance bridge network having a number of resistors connected in series and in series-parallel, said resistors being arranged as a ladder network, switch means shunting certain groups of resistors of said ladder .network, said frequency generator output being connected to said network at various predetermined points between resistors during screening of a patient to give a desired decibel hearing response to a patient, energized masking generator means, an interrupter switch means connecting said masking means to said bridge network means during diagnosis of a patient, a plurality of transducers connected to said network, attenuation switch means inductively and selectively connecting the output of said frequency generator to said bridge attenuation network and multiple switching means for selectively energizing and nullifying the signals from said oscillator to designated transducers during screening and diagnosing the hearing impairments of a patient.

5. An audiometer circuit for determining the hearing impairments personnel comprising a full-wave rectifier energizing circuit, a phase-shift oscillator circuit including a separate frequency determining circuit for each frequency and having an audio signal output, said oscillator circuit being selectively and operably connected to the output of said energizing circuit, variable impedance matching means inductively coupled to the output of said oscillator circuit, calibration means included in each frequency determining circuit for the oscillator circuit, said variable impedance matching means cooperating by mechanical coupling with said calibration means of said oscillator circuit, a plurality of conductive earphone transducers, variable resistive type attenuation circuit means connecting said variable impedance means to said receivers, switch means selectively cooperating with said attenuator means and oscillator to vary the intensity of said oscillator audio signal to said conductive earphone transducer, and masking means energized by said rectifier circuit and selectively connected to said transducers to mask the oscillator signal during diagnosis.

6. An audioimeter comprising a full-wave voltage controlled rectifier energizing circuit, phase-shift oscillator means including pentode tube means with RC network circuit means providing a plurality of variable frequency steps connected between the input electrode means and output electrode means of said pentode tube means, said network permitting energy feedback between the anode and grid terminals of said tube, said oscillation circuit being selectively and operably connected to the output of said energizing circuit, variable impedance matching means inductively coupled to the output of said oscillator circuit, a first calibration means included in said RC network for calibrating the oscillator circuit, a second calibration means in the impedance matching circuit being mechanically and selectively coupled to said first calibration means included in said RC network of said oscillator circuit, a plurality of conductive transducers having selective switch means, variable resistive attenuation circuit means selectively connecting said variable impedance matching means to said transducers to vary the intensity of said audio output signal in decibel units to said transducers, and energized masking means connected to said attenuation means to nullify said oscillator signal to designated transducers during diagnosis.

7. A diagnostic audiometer comprising phase-shift oscillator means including a plurality of frequency determining networks, energized frequency masking generator means, audio transformer means inductively coupled to the output of said oscillator means, a plurality of impedance matching network means selectively connected to the output of said transformer means including variable resistive calibrating means, step attenuating resistive bridge network means operably connected to the output of said impedance network means, selective cooperating mechanical switching means operably connecting said masking means, frequency and impedance matching network with a selected frequency determining network, plural air and bone transducer means connected to the output of said attenuation means, full-wave rectification means for energizing said audiometer, and selective switch means adapted to block out the signal transferred by bone conduction by connecting said generator to said transducers.

References Cited by the Examiner UNITED STATES PATENTS 2,232,779 I 2/1941 Fletcher 1791 2,753,397 7/1956 Zwislocki 1791 2,848,539 8/1958 Allison 179-1 3,091,234 5/1963 Jerger 179-1 ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner. 

1. AN AUDIOMETER CIRCUIT FOR SCREENING AND DIAGNOSTIC USE COMPRISING PHASE-SHIFT OSCILLATOR MEANS INCLUDING A PLURALITY OF FREQUENCY DETERMINING NETWORKS, FREQUENCY MASKING GENERATOR MEANS, AUDIO TRANSFORMER MEANS CONNECTED TO SAID OSCILLATOR MEANS, STEP ATTENUATING RESISTIVE NETWORK MEANS SELECTIVELY CONNECTED TO SAID AUDIO TRANSFORMER MEANS, A PLURALITY OF TRANSDUCERS SELECTIVELY CONNECTED TO SAID ATTENUATION MEANS, SELECTIVE AND INTERRUPTING SWITCH MEANS CONNECTING SAID MASKING MEANS TO SAID SELECTED TRANSDUCER MEANS TO MULLIFY THE OSCILLATOR SIGNAL 